Gas diffusion electrode suitable for use in carbon dioxide electrolyzer and membrane electrode assembly including said gas diffusion electrode

ABSTRACT

Gas diffusion electrode suitable for use in a carbon dioxide electrolyzer and membrane electrode assembly including same. In one embodiment, the gas diffusion electrode may include a gas diffusion layer and a catalyst layer. The gas diffusion layer may be a porous, hydrophobic structure. The gas diffusion layer may have a set of pores designed for gas transport through the gas diffusion layer and may also have a set of openings designed for water drainage and/or water pressure relief. The gas transport pores are smaller in size than the water drainage openings. The catalyst layer may be positioned on a side of the gas diffusion layer and preferably does not cover the openings designed for water drainage and/or water pressure relief. The gas diffusion layer may or may not be electron-conductive. If the gas diffusion layer is non-conductive, the gas diffusion electrode may further include an electron conductive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. 119(e) ofU.S. Provisional Patent Application No. 63/352,040, inventors TianyuZhang et al., filed Jun. 14, 2022, the disclosure of which isincorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-EE0009421awarded by the Department of Energy, Energy Efficiency and RenewableEnergy (DOE EERE). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to carbon dioxide electrolyzersand relates more particularly to a novel gas diffusion electrodesuitable for use in a carbon dioxide electrolyzer, to a membraneelectrode assembly comprising said gas diffusion electrode, to a carbondioxide electrolyzer comprising said membrane electrode assembly, and tomethods for fabricating said gas diffusion electrode, said membraneelectrode assembly, and said carbon dioxide electrolyzer.

The efficient electrochemical conversion of carbon dioxide into valuablecarbon-based fuels and chemicals is desirable both for controllingcarbon emissions and for storage of renewable electricity. In manyinstances, the electrochemical conversion of carbon dioxide intovaluable products is accomplished using a carbon dioxide electrolyzer.Typically, such a carbon dioxide electrolyzer includes a membraneelectrode assembly. The membrane electrode assembly, in turn, oftenincludes an ion exchange membrane positioned between a cathode and ananode. In order to promote a reaction rate that is suitable for carbondioxide conversion on an industrial scale, the cathode is typically inthe form of a gas diffusion electrode. Such a gas diffusion electrodetypically includes a catalyst layer and a gas diffusion layer. Thecatalyst layer functions as the reaction zone where the electrochemicalreduction of carbon dioxide actually occurs whereas the gas diffusionlayer, which is typically a porous structure, functions as the masstransport pathway for supplying the carbon dioxide reactant to thecatalyst layer. In many cases, the gas diffusion layer is made of acarbon material in order to provide a high electron conductivity, whichis necessary for the electrochemical reaction to proceed. Because thecarbon material typically tends to be hydrophilic, which, as discussedbelow, is undesirable, the surface of the carbon material that is usedto make the gas diffusion layer is commonly coated with a fluorinatedmaterial to increase the hydrophobicity and gas transport efficiency ofthe gas diffusion layer.

Ideally, a membrane electrode assembly comprising a gas diffusionelectrode is designed to minimize electrolyzer resistance and toincrease energy efficiency. However, a membrane electrode assemblycomprising a conventional gas diffusion electrode as described above issusceptible to low carbon dioxide transport efficiency during long-termoperation due to one or more of the following reasons: (1) conventionalcarbon-based gas diffusion layers gradually lose hydrophobicity overtime, thereby becoming deficient in preventing flooding of the cathodecatalyst layer with water; (2) water pressure at the cathode catalystlayer gradually builds up as water migrates from the anode side to thecathode side, eventually breaking the pores of the gas diffusionelectrode; and (3) the hydrogen evolution reaction, which competes withthe reduction of carbon dioxide at the cathode catalyst layer, turnssome of the liquid water present at the cathode into hydrogen gas,resulting in a net fluid flux from the catalyst layer to the flowchannel of the gas diffusion layer, thereby hindering the diffusion ofcarbon dioxide towards the catalyst layer.

One approach that has been taken to addressing the above-described gastransport efficiency problem has been to provide a gas diffusionelectrode that comprises an ePTFE (expanded polytetrafluoroethylene)membrane, which is a porous hydrophobic structure. In such a gasdiffusion electrode, a catalyst layer is loaded on top of the ePTFEmembrane, followed by coating a layer of conductive carbon material forelectron conduction. Consequently, as can be appreciated, a gasdiffusion electrode comprising an ePTFE membrane of the above-describedtype decouples the gas transport and electron conduction functions sincethe carbon dioxide reactant diffuses through the ePTFE membrane whereaselectrons are conducted through the conductive carbon material coating.Because, unlike conventional carbon-based gas diffusion membranes, thehydrophobicity of an ePTFE membrane does not wane over time, a gasdiffusion electrode employing an ePTFE membrane intrinsically solves thehydrophobicity degradation problem of conventional gas diffusionelectrodes. However, the problems of water pressure building up in thecatalyst layer and of net gas flux from the cathode layer to the flowchannel due to the hydrogen evolution reaction have not been solved.These problems can have a significant impact on the amount of carbondioxide that reaches the cathode layer and, thus, can significantlylimit the amount of carbon dioxide that is electrochemically converted.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel gasdiffusion electrode that is suitable for use in a carbon dioxideelectrolyzer.

It is another object of the present invention to provide a gas diffusionelectrode as described above that overcomes at least some of theshortcomings associated with at least some existing gas diffusionelectrodes.

It is still another object of the present invention to provide a gasdiffusion electrode as described above that is easy to manufacture andeasy to use.

Therefore, according to one aspect of the invention, there is provided agas diffusion electrode suitable for use in a carbon dioxideelectrolyzer, the gas diffusion electrode comprising (a) a gas diffusionlayer, the gas diffusion layer comprising a porous hydrophobic structurehaving a pair of opposing sides, the gas diffusion layer comprising aset of one or more pores designed for gas transport through the gasdiffusion layer and a set of one or more openings designed for waterdrainage through the gas diffusion layer, wherein the one or more poresare comparatively smaller in size and the one or more openings arecomparatively larger in size; and (b) a catalyst layer, the catalystlayer being disposed on one of the pair of opposing sides.

In a more detailed feature of the invention, the catalyst layer may notcover the one or more openings of the gas diffusion layer.

In a more detailed feature of the invention, the gas diffusion layer maybe electron conductive.

In a more detailed feature of the invention, the gas diffusion layer maycomprise a material selected from the group consisting of a metal orcarbon mesh, woven or felt material, paper, or disc, a porous graphitefilm, a metal foam, a metal sinter, and combinations thereof.

In a more detailed feature of the invention, the gas diffusion layer mayfurther comprise a hydrophobic coating.

In a more detailed feature of the invention, the gas diffusion layer maybe electron non-conductive, and the gas diffusion electrode may furthercomprise an electron conductive layer.

In a more detailed feature of the invention, the gas diffusion layer maycomprise one of a hydrophobic porous polymer membrane, a hydrophobicpolymer-woven material or paper, and combinations thereof.

In a more detailed feature of the invention, the gas diffusion layer maycomprise one of a porous ePTFE (expanded polytetrafluoroethylene)membrane and a porous PVDF (polyvinylidene difluoride) membrane.

In a more detailed feature of the invention, the electron conductivelayer may be a porous structure made of at least one of carbon and ametal.

In a more detailed feature of the invention, each opening of the set ofone or more openings may extend entirely through the gas diffusion layerin a direct fashion from one of the opposing sides to the other of theopposing sides.

In a more detailed feature of the invention, the set of one or moreopenings may comprise a plurality of openings that are uniformlyarranged.

In a more detailed feature of the invention, the set of one or moreopenings may comprise a plurality of openings that are randomlyarranged.

In a more detailed feature of the invention, the set of one or moreopenings may comprise a plurality of regularly-shaped openings.

In a more detailed feature of the invention, the regularly-shapedopenings may be circular openings.

In a more detailed feature of the invention, the circular openings mayhave a diameter that is larger than about 30 μm.

In a more detailed feature of the invention, the regularly-shapedopenings may be square openings.

In a more detailed feature of the invention, the regularly-shapedopenings may be rectangular slits.

In a more detailed feature of the invention, the rectangular slits maybe intersecting lines arranged in a rectangular grid pattern.

In a more detailed feature of the invention, the rectangular slits maybe non-intersecting lines arranged in a rectangular grid pattern.

In a more detailed feature of the invention, the set of one or moreopenings may comprise a plurality of irregularly-shaped openings.

In a more detailed feature of the invention, the irregularly-shapedopenings may be slits.

In a more detailed feature of the invention, the set of one or moreopenings may comprise a plurality of slits, and the slits may have alength that exceeds about 1 millimeter.

In a more detailed feature of the invention, the gas diffusion layer mayconsist of a single piece of material.

In a more detailed feature of the invention, the gas diffusion layer maycomprise a plurality of distinct pieces separated from one another byspaces, and the spaces may form the one or more openings of the gasdiffusion layer.

In a more detailed feature of the invention, the plurality of distinctpieces may be arranged uniformly in a single plane in a rectangular gridpattern.

In a more detailed feature of the invention, the plurality of distinctpieces may be arranged randomly in a single plane.

According to another aspect of the present invention, there is provideda novel membrane electrode assembly suitable for use in a carbon dioxideelectrolyzer, the membrane electrode assembly comprising (a) an ionexchange membrane, the ion exchange membrane having an anode side and acathode side; (b) an anode coupled to the anode side of the ion exchangemembrane; and (c) the above-described gas diffusion electrode coupled tothe cathode side of the ion exchange membrane.

In a more detailed feature of the invention, the gas diffusion layer maybe electron conductive, and the membrane electrode assembly may furthercomprise a water barrier layer positioned between the catalyst layer andthe ion exchange membrane.

In a more detailed feature of the invention, the water barrier layer maybe a porous structure comprising one or more materials selected from thegroup of hydrophobic polymer particles, porous hydrophobic polymerparticles, hydrophobic polymer fibers, and hydrophobic polymer pellets.

In a more detailed feature of the invention, the water barrier layer mayhave a thickness in a range of about 200 nm to about 1 mm.

In a more detailed feature of the invention, the gas diffusion layer maybe electron conductive, and the membrane electrode assembly may furthercomprise a water capillary membrane positioned between the catalystlayer and the ion exchange membrane.

In a more detailed feature of the invention, the water capillarymembrane may be a porous and hydrophilic structure having a thickness ina range of about 30 micrometers to about 1 millimeter.

In a more detailed feature of the invention, the membrane electrodeassembly may further comprise a water barrier layer positioned betweenthe water capillary membrane and the catalyst layer.

In a more detailed feature of the invention, the gas diffusion layer maybe electron non-conductive, and the membrane electrode assembly mayfurther comprise an electron conductive layer positioned between thecatalyst layer and the ion exchange membrane.

In a more detailed feature of the invention, the electron conductivelayer may be a porous structure comprising one or more materialsselected from the group of carbon, copper, iron, stainless steel,silver, gold, nickel, aluminum, molybdenum, zinc, titanium, brass, and ametal alloy.

In a more detailed feature of the invention, the electron conductivelayer may have a thickness in a range from about 30 micrometers to about2 millimeters.

In a more detailed feature of the invention, the membrane electrodeassembly may further comprise a water capillary membrane positionedbetween the ion exchange membrane and the electron conductive layer.

According to yet another aspect of the invention, there is provided acarbon dioxide electrolyzer, the carbon dioxide electrolyzer comprising(a) the membrane electrode assembly as described above; and (b) avoltage source, the voltage source operatively coupled to the membraneelectrode assembly.

The present invention is also directed at novel methods for fabricatinga gas diffusion electrode, a membrane electrode assembly, and a carbondioxide electrolyzer.

For purposes of the present specification and claims, various relationalterms like “top,” “bottom,” “proximal,” “distal,” “upper,” “lower,”“front,” and “rear” may be used to describe the present invention whensaid invention is positioned in or viewed from a given orientation. Itis to be understood that, by altering the orientation of the invention,certain relational terms may need to be adjusted accordingly.

Additional objects, as well as aspects, features, and advantages, of thepresent invention will be set forth in part in the description whichfollows, and in part will be obvious from the description or may belearned by practice of the invention. In the description, reference ismade to the accompanying drawings which form a part thereof and in whichis shown by way of illustration various embodiments for practicing theinvention. The embodiments will be described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and that structuralchanges may be made without departing from the scope of the invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is best definedby the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are hereby incorporated into andconstitute a part of this specification, illustrate various embodimentsof the invention and, together with the description, serve to explainthe principles of the invention. The drawings are not necessarilydrawing to scale, and certain components may have undersized and/oroversized dimensions for purposes of explication. In the drawingswherein like reference numeral represents like parts:

FIG. 1 is a simplified side view of one embodiment of an electrochemicalcell constructed according to the present invention;

FIG. 2 is a simplified fragmentary section view of the membraneelectrode assembly shown in FIG. 1 ;

FIGS. 3A and 3B are top and bottom views, respectively, of the gasdiffusion layer shown in FIG. 2 ;

FIG. 4 is a schematic diagram illustrating the mass transport mechanismin the membrane electrode assembly of FIG. 2 , the anode of the membraneelectrode assembly not being shown for simplicity;

FIGS. 5A through 5G are top views of alternative gas diffusion layers tothe gas diffusion layer of FIG. 3A;

FIGS. 6A and 6B are top views of two additional alternative gasdiffusion layers to the gas diffusion layer of FIG. 3A;

FIGS. 7A and 7B are top views of yet another two alternative gasdiffusion layers to the gas diffusion layer of FIG. 3A;

FIG. 8 is a top perspective view of an alternative gas diffusionelectrode to the gas diffusion electrode of FIG. 2 , the alternative gasdiffusion electrode comprising the gas diffusion layer of FIG. 7A;

FIG. 9 is a simplified fragmentary section view of a first alternativemembrane electrode assembly to that shown in FIG. 2 ;

FIG. 10 is a schematic diagram illustrating the mass transport mechanismin the membrane electrode assembly of FIG. 9 , the anode of the membraneelectrode assembly and the intrinsic pores of the gas diffusion layernot being shown for simplicity;

FIG. 11 is a simplified fragmentary section view of a second alternativemembrane electrode assembly to that shown in FIG. 2 ;

FIG. 12 is a schematic diagram illustrating the mass transport mechanismin the membrane electrode assembly of FIG. 11 , the anode of themembrane electrode assembly and the intrinsic pores of the gas diffusionlayer not being shown for simplicity;

FIG. 13 is a simplified fragmentary section view of a third alternativemembrane electrode assembly to that shown in FIG. 2 ;

FIG. 14 is a schematic diagram illustrating the mass transport mechanismin the membrane electrode assembly of FIG. 13 , the anode of themembrane electrode assembly and the intrinsic pores of the gas diffusionlayer not being shown for simplicity;

FIG. 15 is a graph depicting the stability test performance for the gasdiffusion electrode of Example 1; and

FIG. 16 is a graph depicting the stability test performance for the gasdiffusion electrode of Example 2.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, the presence of excess water at the cathode catalystlayer of a carbon dioxide electrolyzer can significantly limit theamount of carbon dioxide that reaches the cathode catalyst layer and,thus, can significantly reduce the amount of carbon dioxide that isreacted by the carbon dioxide electrolyzer. The present invention isdirected, at least in part, at a novel gas diffusion electrode that maybe suitable for ameliorating the foregoing problem. In particular, aswill be discussed further below, in at least some embodiments, the gasdiffusion electrode of the present invention may be designed as apressure relief matrix electrode. More specifically, in at least oneembodiment, the gas diffusion electrode may comprise a gas diffusionlayer and a catalyst layer, wherein the gas diffusion layer may be aporous structure that additionally includes one or more openings orchannels that extend perpendicularly through the entire thickness of thegas diffusion layer in a straight-line fashion, thereby creatingspecific areas to drain water from an adjacent portion of the catalystlayer and, thus, providing water pressure relief in the adjacent areasof the catalyst layer. In at least one embodiment, the gas diffusionlayer may comprise an electron-conductive material, whereas, in at leastone embodiment, the gas diffusion layer may comprise an electronnon-conductive material. The present invention is also directed, atleast in part, at a membrane electrode assembly that includes a gasdiffusion electrode as described above and is further directed, at leastin part, at an electrochemical cell, such as a carbon dioxideelectrolyzer, that includes such a membrane electrode assembly.

Referring now to FIG. 1 , there is shown a simplified side view of oneembodiment of an electrochemical cell constructed according to thepresent invention, the electrochemical cell being represented generallyby reference numeral 11. Details of electrochemical cell 11 that arediscussed elsewhere in this application or that are not critical to anunderstanding of the invention may be omitted from FIG. 1 and/or fromthe accompanying description herein or may be shown in FIG. 1 and/ordescribed herein in a simplified manner.

Electrochemical cell 11 is preferably an electrolytic cell and morepreferably is a carbon dioxide electrolyzer, which may be designed, forexample, to convert carbon dioxide into carbon-containing materials,such as, but not limited to, ethylene. Electrochemical cell 11 maycomprise a membrane electrode assembly 13, which is discussed in greaterdetail below. Electrochemical cell 11 may further comprise a firstcurrent collector 14, a second current collector 16, a first endplate18, a second endplate 20, and a voltage source 22, all of which may beconventional. First current collector 14 and second current collector 16may be coupled to membrane electrode assembly 13 on opposite sidesthereof. In addition, first current collector 14 and second currentcollector 16 may be electrically coupled to voltage source 22 throughleads 24-1 and 24-2, respectively. First endplate 18 may be positionedabove and coupled to first current collector 14, and second endplate 20may be positioned below and coupled to second current collector 16.

Although not shown, electrochemical cell 11 may further comprise variousgaskets, frames, and gas transport structures of the type conventionallyused in electrochemical cells, particularly carbon dioxideelectrolyzers.

Where, for example, electrochemical cell 11 is a carbon dioxideelectrolyzer, carbon dioxide may be delivered to the cathode side ofmembrane electrode assembly 13, and a suitable reductant, such as apotassium hydroxide solution, may be delivered to the anode side ofmembrane electrode assembly 13. As a result of electrolysis, the carbondioxide supplied to electrochemical cell 11 may be reduced to ethyleneor the like.

Referring now to FIG. 2 , there is shown a simplified section view ofmembrane electrode assembly 13. Details of membrane electrode assembly13 that are discussed elsewhere in this application or that are notcritical to an understanding of the invention may be omitted from FIG. 2and/or from the accompanying description herein or may be shown in FIG.2 and/or described herein in a simplified manner.

Membrane electrode assembly 13 may comprise an ion exchange membrane 15,an anode 17 positioned on one side of ion exchange membrane 15, and acathode 19 positioned on the opposite side of ion exchange membrane 15.Ion exchange membrane 15, which functions as a separator and ionconductor between anode 17 and cathode 19, may be an ion exchangemembrane of the type typically used in a carbon dioxide electrolyzer.More specifically, ion exchange membrane 15 may be, for example, aproton exchange membrane, an anion exchange membrane, or a bipolarmembrane, with anion exchange membranes and bipolar membranes beingpreferred and with anion exchange membranes being particularlypreferred.

Anode 17 may comprise an anode of the type conventionally used in acarbon dioxide electrolyzer. For example, the anode may be a piece ofmetal foam that is catalytically active for anode reactions, or theanode may be a piece of metal foam having a surface on which activecatalysts are deposited, or the anode may be a gas diffusion electrodecomprising a porous transport layer and a catalyst layer.

Cathode 19 may comprise a gas diffusion electrode and, thus, maycomprise a gas diffusion layer 21 and a catalyst layer 23. (Catalystlayer 23, which may be conventional, may be deposited or otherwisepositioned directly on top of gas diffusion layer 21 in the conventionalmanner. For example, catalyst layer 23 may be a layer of catalystparticles coated on top of gas diffusion layer 21 to a thickness rangingfrom 100 nm to 10 μm. Alternatively, catalyst layer 23 may comprise, forexample, catalyst particles loaded onto a substrate layer, which arethen coated together onto gas diffusion layer 21.) Gas diffusion layer21, which is also shown separately in FIGS. 3A and 3B, may comprise anelectron-conductive gas diffusion layer and, as will be discussedfurther below, may be similar in certain respects to any type ofelectron-conductive gas diffusion layer, whether conventional orotherwise. Examples of materials suitable for use in making gasdiffusion layer 21 may comprise, for example, a porouselectron-conductive material, such as a metal or carbon mesh, woven orfelt material, paper, or disc, a porous graphite film, a metal foam, ametal sinter, or combinations thereof. As such, gas diffusion layer 21may include a plurality of intrinsic pores or channels 26 for gastransport therethrough, such as transport of carbon dioxide upwardlythrough gas diffusion layer 21 to catalyst layer 23. (For simplicity,the intrinsic pores or channels 26 of gas diffusion layer 21 are notshown in FIG. 2 .) In the present embodiment, gas diffusion layer 21 maybe a unitary member consisting of a single piece of material; however,it is to be understood that, alternatively, gas diffusion layer 21 couldbe made by joining together a plurality of separate pieces of materialto form a coherent structure.

Where gas diffusion layer 21 is made of carbon and/or metal, gasdiffusion layer 21 may be of limited hydrophobicity. Such limitedhydrophobicity is undesirable as it may cause and/or exacerbate one ormore of the problems discussed above. To improve the hydrophobicity ofgas diffusion layer 21, gas diffusion layer 21 may be coated or treatedin the conventional manner with one or more hydrophobic materials.

Gas diffusion layer 21 may differ from conventional electron-conductivegas diffusion layers in that gas diffusion layer 21 may additionally beprovided with one or more openings (or channels) 27 that extend throughthe entire thickness of gas diffusion layer 21, preferablyperpendicularly through gas diffusion layer 21 in a direct (i.e.,straight-line) fashion from a top surface 28-1 of gas diffusion layer 21to a bottom surface 28-29 of gas diffusion layer 21. In the presentembodiment, openings 27 may be in the shape of circular holes and may beuniformly arranged throughout gas diffusion layer 21 in a linearpattern; however, as will be discussed further below, openings 27 neednot be in the form of circular holes and/or need not be arranged in alinear or uniform pattern. In any event, in contrast with conventionalgas diffusion electrodes, gas diffusion layer 21 may be constructed sothat the reaction zone of catalyst layer 23 may effectively be dividedby openings 27 into a plurality of sub-reaction zones, with an opening27 being provided adjacent to one or more of said sub-reaction zones. Inthis manner, by providing a plurality of openings 27 in gas diffusionlayer 21, water may be drained from the adjacent sub-reaction zones. Assuch, openings 27 may serve as water drainage or pressure releasechannels. More specifically, the water breakthrough pressure at openings27 is low; consequently, due to openings 27, water pressure does notbuild up in the nearby sub-reaction zones of catalyst layer 23.

It is to be understood that the number, size, and distribution ofopenings 27 shown in FIGS. 3A and 3B is merely illustrative and that agreater or lesser number of openings 27 of greater or lesser size may beprovided in any sort of arrangement.

Openings 27 may be formed in gas diffusion layer 21 by a variety ofcutting techniques, such as, but not limited to, shape die cutting andlaser cutting. Openings 27 may vary in size from the nanometer scale tothe millimeter scale; preferably, openings 27 are larger in diameterthan the intrinsic pores 26 of gas diffusion layer 21. Where, as in thepresent embodiment, openings 27 are circular or similar in shape,openings 27 preferably have a diameter that is larger than about 30 μm.By contrast, where openings 27 have a slit shape, the width of openings27 can be down to the nanometer scale, but the length should be longerthan about 1 millimeter.

In addition, openings 27 may be fewer in number than the intrinsic pores26 of gas diffusion layer. In fact, in theory, there may be as few asone opening 27. The size of intrinsic pores 26 may be no more than 10 μmin diameter. Additionally, the porosity of gas diffusion layer 21attributable to intrinsic pores 26 may be more than 50%.

Referring back now to FIG. 2 , membrane electrode assembly 13 mayfurther comprise a water barrier layer 29, wherein water barrier layer29 may be positioned between ion exchange membrane 15 and cathode 19and, more specifically, may be applied directly to the top surface ofcatalyst layer 23 (i.e., the surface of catalyst layer 23 that isopposite gas diffusion layer 21). Water barrier layer 29 may consist ofor comprise one or more porous materials possessing a high degree ofhydrophobicity. Water barrier layer 29 may have a thickness that variesfrom the nanometer scale to hundreds of micrometers (e.g., about 200 nmto about 1 mm), depending on its porosity and hydrophobicity. Thehydrophobicity of water barrier layer 29 may come from the properties ofthe material used to make water barrier layer 29 or from a hydrophobiccoating applied to the material used to make water barrier layer 29.Water barrier layer 29 may comprise one or more of hydrophobic polymerparticles, porous hydrophobic polymer particles, hydrophobic polymerfibers, and hydrophobic polymer pellets. For example, water barrierlayer 29 may be made by coating a layer of 100 nm PTFE particles oncatalyst layer 23. The function of water barrier layer 29 is towithstand part of the water pressure in the ion exchange membrane 15 andto reduce the quantity of water going into catalyst layer 23.Notwithstanding the above, the inclusion of water barrier layer 29 inmembrane electrode assembly 13 is optional. While providing someresistance to water diffusion, water barrier layer 29 may still allowsome water to pass therethrough.

Catalyst layer 23 and water barrier layer 29 are preferably absent inthe areas above openings 27 of gas diffusion layer 21. By contrast,catalyst layer 23 and water barrier layer 29 preferably are present inthe remaining areas above gas diffusion layer 21.

It is to be understood that, although each of ion exchange membrane 15,anode 17, gas diffusion layer 21, catalyst layer 23, and water barrierlayer 29 is shown in FIG. 2 as a monolithic layer of material, one ormore of said layers may comprise a composite layer or laminate materialcomprising a plurality of distinct sublayers.

Referring now to FIG. 4 , there is schematically shown the masstransport mechanism of membrane electrode assembly 13. As can be seen,carbon dioxide may flow upwardly through gas diffusion layer 21 via itsintrinsic pores, which pores are not shown in FIG. 4 for simplicity, tocatalyst layer 23. In addition, electrons may be conducted upwardly viathe conductive structural framework of gas diffusion layer 21 tocatalyst layer 23. Much of the water in ion exchange membrane 15 may bekept from passing from ion exchange membrane 15 to catalyst 23 by waterbarrier layer 29; instead, such water may be diverted to areas alignedwith openings 27, and such water may drain through openings 27. Inaddition, at least some of the water that passes through water barrierlayer 29 into catalyst layer 23 may drain into openings 27.

Referring now to FIGS. 5A through 5G, there are shown top views ofalternative gas diffusion layers that may be used in place of gasdiffusion layer 21 to form cathode 19 of membrane electrode assembly 13.For simplicity, details of such gas diffusion layers that are discussedelsewhere in this application or that are not critical to anunderstanding of the invention may be omitted from one or more of FIGS.5A through 5G and/or from the accompanying description herein or may beshown in one or more of FIGS. 5A through 5G and/or described herein in asimplified manner. More specifically, in FIG. 5A, there is shown a gasdiffusion layer 51. Gas diffusion layer 51 may differ from gas diffusionlayer 21 in that, whereas gas diffusion layer 21 may include openings(or channels) 27 in the shape of circular holes that are uniformlyarranged in a linear pattern, gas diffusion layer 51 may, instead,include openings (or channels) 53 in the shape of square holes that areuniformly arranged in a linear pattern. In FIG. 5B, there is shown a gasdiffusion layer 57. Gas diffusion layer 57 may differ from gas diffusionlayer 21 in that, whereas gas diffusion layer 21 may include openings(or channels) 27 in the shape of circular holes that are uniformlyarranged in a linear pattern, gas diffusion layer 57 may, instead,include openings (or channels) 59 in the shape of irregularly-shapedholes that are uniformly arranged in a linear pattern. In FIG. 5C, thereis shown a gas diffusion layer 61. Gas diffusion layer 61 may differfrom gas diffusion layer 21 in that, whereas gas diffusion layer 21 mayinclude openings (or channels) 27 in the shape of circular holes thatare uniformly arranged in a linear pattern, gas diffusion layer 61 may,instead, include openings (or channels) 63 in the shape ofirregularly-shaped slits that are generally uniformly arranged in alinear pattern. In FIG. 5D, there is shown a gas diffusion layer 67. Gasdiffusion layer 67 may differ from gas diffusion layer 21 in that,whereas gas diffusion layer 21 may include openings (or channels) 27 inthe shape of circular holes that are uniformly arranged in a linearpattern, gas diffusion layer 67 may, instead, include openings (orchannels) 69 in the shape of circular holes that are randomly arranged.In FIG. 5E, there is shown a gas diffusion layer 71. Gas diffusion layer71 may differ from gas diffusion layer 21 in that, whereas gas diffusionlayer 21 may include openings (or channels) 27 in the shape of circularholes that are uniformly arranged in a linear pattern, gas diffusionlayer 71 may, instead, include openings (or channels) 73 in the shape ofsquare holes that are randomly arranged. In FIG. 5F, there is shown agas diffusion layer 77. Gas diffusion layer 77 may differ from gasdiffusion layer 21 in that, whereas gas diffusion layer 21 may includeopenings (or channels) 27 in the shape of circular holes that areuniformly arranged in a linear pattern, gas diffusion layer 77 may,instead, include openings (or channels) 79 in the shape ofirregularly-shaped holes that are randomly arranged. In FIG. 5G, thereis shown a gas diffusion layer 81. Gas diffusion layer 81 may differfrom gas diffusion layer 21 in that, whereas gas diffusion layer 21 mayinclude openings (or channels) 27 in the shape of circular holes thatare uniformly arranged in a linear pattern, gas diffusion layer 81 may,instead, include openings (or channels) 83 in the shape ofirregularly-shaped slits that are randomly arranged.

It is to be understood that, although the water drainage orpressure-relief openings or channels of each of gas diffusion layers 51,57, 61, 67, 71, 77 and 81 are similar or identical in size and shape tothe other water drainage or pressure-relief openings or channels of thesame gas diffusion layer, this need not be the case as a single gasdiffusion layer may include a plurality of water drainage orpressure-relief openings or channels of different sizes and/or shapes,and such water drainage or pressure-relief openings or channels may bearranged in an ordered pattern or in a random pattern.

Referring now to FIGS. 6A and 6B, there are shown top views of twoadditional alternative gas diffusion layers that may be used in place ofgas diffusion layer 21 in membrane electrode assembly 13. Forsimplicity, details of such gas diffusion layers that are discussedelsewhere in this application or that are not critical to anunderstanding of the invention may be omitted from one or more of FIGS.6A and 6B and/or from the accompanying description herein or may beshown in one or more of FIGS. 6A and 6B and/or described herein in asimplified manner. More specifically, in FIG. 6A, there is shown a gasdiffusion layer 91. Gas diffusion layer 91 may differ from gas diffusionlayer 21 in that, whereas gas diffusion layer 21 may include openings(or channels) 27 in the shape of circular holes that are uniformlyarranged in a linear pattern, gas diffusion layer 91 may, instead,include openings (or channels) 93 and 94 in the shape ofnon-intersecting horizontal and vertical lines, respectively, openings93 and 94 being arranged in a rectangular grid pattern. In FIG. 6B,there is shown a gas diffusion layer 97. Gas diffusion layer 97 maydiffer from gas diffusion layer 21 in that, whereas gas diffusion layer21 may include openings (or channels) 27 in the shape of circular holesthat are uniformly arranged in a linear pattern, gas diffusion layer 97may, instead, include openings (or channels) 98 and 99 in the shape ofintersecting horizontal and vertical lines, respectively, openings 98and 99 being arranged in a rectangular grid pattern.

Referring now to FIGS. 7A and 7B, there are shown top views of twoadditional alternative gas diffusion layers that may be used in place ofgas diffusion layer 21 in membrane electrode assembly 13. Forsimplicity, details of such gas diffusion layers that are discussedelsewhere in this application or that are not critical to anunderstanding of the invention may be omitted from one or more of FIGS.7A and 7B and/or from the accompanying description herein or may beshown in one or more of FIGS. 7A and 7B and/or described herein in asimplified manner. More specifically, in FIG. 7A, there is shown a gasdiffusion layer 101. Gas diffusion layer 101 may comprise a plurality ofgas diffusion layer pieces 103. Each of gas diffusion layer pieces 103may be similar or identical in composition and structure to a gasdiffusion layer of the electron-conductive type but may be reduced infootprint thereto. In addition, each of gas diffusion layer pieces 103may have a generally rectangular footprint, each of gas diffusion layerpieces 103 may be similar or identical to one another in size, shape andcomposition, and each of gas diffusion layer pieces 103 may be arrangedin a single plane in a rectangular grid pattern such that each gasdiffusion layer piece 103 is spaced apart from each of its neighboringgas diffusion layer pieces 103 by a space (or channel) 105. In thismanner, spaces 105 may function like openings 98 and 99 of gas diffusionlayer 97 to permit water to pass through gas diffusion layer 101.Although not shown, gas diffusion layer pieces 103 may be mounted on asuitable substrate or may otherwise be fixed in space relative to oneanother to maintain the desired spacing between gas diffusion layerpieces 103. Also, although, in the present embodiment, gas diffusionlayer pieces 103 of gas diffusion layer 101 are said to be identical toone another in size, shape, and composition, some of gas diffusion layerpieces 103 could be made to be different than other of gas diffusionlayer pieces 103 in one or more of size, shape, and composition.

In FIG. 7B, there is shown a gas diffusion layer 111. Gas diffusionlayer 111 may comprise a plurality of gas diffusion layer pieces 113,wherein each of gas diffusion layer pieces 113 may be similar oridentical to gas diffusion layer pieces 103 of gas diffusion layer 101.Gas diffusion layer 111 may differ from gas diffusion layer 101 in that,whereas gas diffusion layer pieces 103 may be arranged in a single planein a rectangular grid pattern, gas diffusion layer pieces 113 may bearranged in a single plane in a non-uniform or random pattern. As aresult, depending on the arrangement of gas diffusion layer pieces 113,the spacing between any two given adjacent gas diffusion layer pieces113 may be uniform or may vary, and the spacing across the entirety ofgas diffusion layer 111 may be non-uniform. Although not shown, gasdiffusion layer pieces 113 may be mounted on a suitable substrate or mayotherwise be fixed in space relative to one another to maintain thedesired spacing between gas diffusion layer pieces 113. Also, although,in the present embodiment, gas diffusion layer pieces 113 of gasdiffusion layer 111 are said to be identical to one another in size,shape, and composition, some of gas diffusion layer pieces 113 could bemade to be different than other of gas diffusion layer pieces 113 in oneor more of size, shape, and composition.

Referring now to FIG. 8 , there is shown a top perspective view of analternative gas diffusion electrode that may be used in place of cathode19 in membrane electrode assembly 13, the alternative gas diffusionelectrode being represented generally by reference numeral 131. Forsimplicity, details of gas diffusion electrode 131 that are discussedelsewhere in this application or that are not critical to anunderstanding of the invention may be omitted from FIG. 8 and/or fromthe accompanying description herein or may be shown in FIG. 8 and/ordescribed herein in a simplified manner. In the present embodiment, gasdiffusion electrode 131 may comprise a gas diffusion layer 133 and acatalyst layer 135. Gas diffusion layer 133 may be similar or identicalto gas diffusion layer 101 and may comprise a plurality of identical gasdiffusion layer pieces 137. Catalyst layer 135, which may be applied tothe top surfaces of gas diffusion layer pieces 137, may be similar oridentical in thickness and/or composition to catalyst layer 23 and maycomprise a plurality of catalyst layer pieces 139. As can beappreciated, each combination of a gas diffusion layer piece 137 and itsassociated catalyst layer piece 139 may be regarded as a gas diffusionelectrode piece 141, with adjacent gas diffusion electrode pieces 141being separated from one another by a space (or channel) 143.

As can be appreciated, one could modify gas diffusion electrode 131 byreplacing gas diffusion layer 133 with gas diffusion layer 111.

Referring now to FIG. 9 , there is shown a simplified section view of afirst alternative membrane electrode assembly to membrane electrodeassembly 13, said first alternative membrane electrode assembly beingrepresented generally by reference numeral 171. Details of membraneelectrode assembly 171 that are discussed elsewhere in this applicationor that are not critical to an understanding of the invention may beomitted from FIG. 9 and/or from the accompanying description herein ormay be shown in FIG. 9 and/or described herein in a simplified manner.For example, the intrinsic pores of gas diffusion layer 21 are not shownin FIG. 9 .

Membrane electrode assembly 171 may be similar in most respects tomembrane electrode assembly 13, wherein the principal difference betweenthe two membrane electrode assemblies may be that membrane electrodeassembly 171 may additionally include a water capillary membrane 173that may be positioned between ion exchange membrane 15 and waterbarrier layer 29. Water capillary membrane 173, which acts as a bufferbetween catalyst layer 23 and ion exchange membrane 15, is preferably aporous and hydrophilic structure that facilitates water flow and removalin a horizontal direction, enabling the excess water to then circumventthe reaction zone and to avoid flooding catalyst layer 23. Watercapillary membrane 173 may be, for example, a piece of cellulose fibersheet or non-woven fiber paper. Water capillary membrane 173 also allowsgas to flow in the horizontal direction. Water capillary membrane 173may have a thickness ranging from about 30 micrometers to about 1millimeter, depending on its porosity and hydrophilicity.

The mass transport mechanism of membrane electrode assembly 171 isschematically shown in FIG. 10 .

As will be discussed further below, although the various gas diffusionlayers described above are said to be electron-conductive, it is to beunderstood that corresponding gas diffusion layers could alternativelybe made of electron non-conductive materials. However, where gasdiffusion layers are made of electron non-conductive materials, a gasdiffusion electrode comprising the non-conductive gas diffusion layermay further comprise an electron conductive layer deposited over thecatalyst layer.

Referring now to FIG. 11 , there is shown a simplified section view of asecond alternative membrane electrode assembly to membrane electrodeassembly 13, said second alternative membrane electrode assembly beingrepresented generally by reference numeral 201. Details of membraneelectrode assembly 201 that are discussed elsewhere in this applicationor that are not critical to an understanding of the invention may beomitted from FIG. 11 and/or from the accompanying description herein ormay be shown in FIG. 11 and/or described herein in a simplified manner.

Membrane electrode assembly 201 may be similar in many respects tomembrane electrode 13. One difference between the two membrane electrodeassemblies may be that, whereas membrane electrode assembly 13 maycomprise a gas diffusion layer 21 that is electron-conductive, membraneelectrode assembly 201 may, instead, comprise a gas diffusion layer 203that is not electron-conductive (i.e., electron non-conductive).Examples of materials that may be used to make gas diffusion layer 203may include hydrophobic porous electron non-conductive materials, suchas, but not limited to, hydrophobic porous polymer membranes, such asporous ePTFE membranes or porous PVDF (polyvinylidene difluoride)membranes, hydrophobic polymer-woven materials or papers, and the like.Gas diffusion layer 203 may comprise a plurality of openings 205 thatmay be similar to any of the various types of water drainage orpressure-relief openings or channels that are described in the presentpatent application. Although, in the present embodiment, gas diffusionlayer 203 is a one-piece structure, it is to be understood that gasdiffusion layer 203 may be made from a plurality of pieces, as in thecases of gas diffusion layers 101 and 111. For simplicity, the intrinsicpores of gas diffusion layer 203 are not shown in FIG. 11 .

Another difference between membrane electrode assembly 201 and membraneelectrode assembly 13 may be that membrane electrode assembly 201 neednot include a water barrier layer like water barrier layer 29. This isparticularly the case where gas diffusion layer 203 is made of amaterial, such as ePTFE, that possesses good hydrophobicity.

Still another difference between membrane electrode assembly 201 andmembrane electrode assembly 13 may be that membrane electrode assembly201 may further comprise an electron conductive layer 207. Electronconductive layer 207 may be necessary since, as noted above, gasdiffusion layer is electron non-conductive. Electron conductive layer207 may be a porous structure and may comprise, for example, one or morematerials like carbon, copper, iron, stainless steel, silver, gold,nickel, aluminum, molybdenum, zinc, titanium, brass, or a metal alloy.Materials with low catalytic activity, such as carbon, may be preferredfor electron conductive layer 207. Examples of carbon materials mayinclude carbon paper, carbon disk, and carbon foam. The thickness ofelectron conductive layer 207 may range from about 30 micrometers toabout 2 millimeters. Conductive carbon particles or carbon blacks can beadded to electron conductive layer 207 to reduce its porosity andenhance in-plane electron conductivity. The conductive carbon particlesmay be treated with hydrophilic polymers to make them more hydrophilic.In addition or alternatively, electron conductive layer 207 may furthercomprise certain amounts of hydrophilic materials or ionomers toincrease its ion conductivity. On the other hand, electron conductivelayer 207 may contain certain amounts of hydrophobic materials toincrease the water breakthrough pressure so that electron conductivelayer 207 can withstand part of the water pressure and reduce thequantity of water going into catalyst layer 23. When electrons areconducted through electron conductive layer 207, water and gas also mayflow horizontally therethrough.

The combination of electron conductive layer 207, catalyst layer 23, andgas diffusion layer 203 may collectively form a cathode 209.

The mass transport mechanism of membrane electrode assembly 201 isschematically shown in FIG. 12 .

Referring now to FIG. 13 , there is shown a simplified section view of athird alternative membrane electrode assembly to membrane electrodeassembly 13, said third alternative membrane electrode assembly beingrepresented generally by reference numeral 301. Details of membraneelectrode assembly 301 that are discussed elsewhere in this applicationor that are not critical to an understanding of the invention may beomitted from FIG. 13 and/or from the accompanying description herein ormay be shown in FIG. 13 and/or described herein in a simplified manner.

Membrane electrode assembly 301 may be similar in many respects tomembrane electrode 201, wherein the principal difference between the twomembrane electrode assemblies may be that membrane electrode assembly301 may further comprise a water capillary membrane 303 positionedbetween ion exchange membrane 15 and electron conductive layer 207.Water capillary membrane 303 may be porous and hydrophilic and may beidentical to water capillary membrane 173 of membrane electrode assembly171. Water capillary membrane 303 may facilitate water flow in ahorizontal direction, thereby enabling excess water to circumvent thereaction zone and avoid flooding catalyst layer 23. Water capillarymembrane 303 also may allow gas to flow in the horizontal direction.Water capillary membrane 303 may be omitted if electron conductive layer207 is thick enough.

The mass transport mechanism of membrane electrode assembly 201 isschematically shown in FIG. 14 .

Some additional comments, features, aspects and/or observations relatingto one or more embodiments of the present invention are provided below.

-   -   According to one aspect of the invention, a Pressure Relief        Matrix Electrode (PRME) is provided that includes a Pressure        Relief Matrix Gas Diffusion Layer (PRMGDL) serving as the gas        diffusion media and a Catalyst Layer (CL) functioning as the        reaction zone.    -   The Pressure Relief Matrix Gas Diffusion Layer is preferably a        porous and hydrophobic membrane, which can be either conductive        or non-conductive. It preferably contains numerous water        pressure relief channels that extend throughout its thickness.    -   The diameters and lengths for the water pressure relief channels        can vary from the nanometer scale to the millimeter scale. If a        water pressure relief channel has a round shape, its diameter        should be larger than about 30 μm. If the water pressure relief        channel has a slit shape, its width can be down to nanometer        scale, but its length should be longer than about 1 millimeter.    -   The water pressure relief channels can have various shapes and        patterns, such as linear or non-linear. A reason for        incorporating multiple holes and slits in the gas diffusion        layer is to create specific areas for draining water originating        from the anode via membrane crossover. The water breakthrough        pressure at these holes and slits is low, preventing water        buildup in the catalyst layer.    -   The water pressure relief channels can be periodically patterned        or randomly distributed, with a preference for uniform        distribution.    -   Various cutting methods like shape die cutting or laser cutting        can be employed to create the water pressure relief channels.    -   Alternatively, the water pressure relief channels can be formed        by assembling multiple small sub-gas diffusion layers together,        leaving gaps between each pair of sub-gas diffusion layers.    -   In the case of a non-conductive gas diffusion layer, it can be        made using hydrophobic porous polymer membranes, such as porous        PTFE or porous PVDF. Hydrophobic polymer-woven materials or        papers can also be utilized.    -   The electron conductive layer can be a piece of regular carbon        paper, metal foam, metal mesh, metal woven, or metal sinter.    -   The gas diffusion electrode may incorporate numerous        through-holes and slits that penetrate its entire thickness due        to the structure of the gas diffusion layer.    -   A membrane electrode assembly (MEA) design may be based on an        electron-conductive pressure relief matrix (PRM) gas diffusion        layer (GDL) that includes four layers and one optional layer        (not including the anode). These five layers may be as        follows: 1) PRM GDL, 2) cathode catalyst layer, 3) optional        water barrier layer, 4) hydrophilic porous media, and 5) ion        exchange membrane.    -   The electron-conductive PRM GDL and the cathode catalyst layer        collectively form a PRME (pressure relief matrix electrode).    -   The aforementioned optional water barrier layer may be a        hydrophobic polymer coating on the surface of the catalyst        layer. While providing some resistance to water diffusion, this        layer may still allow water to pass through.    -   The thickness of the water barrier layer may vary from about 200        nanometers to about 1 millimeter, depending on its porosity and        hydrophobicity.    -   The water barrier layer may comprise one or more of hydrophobic        polymer particles, porous hydrophobic polymer particles,        hydrophobic polymer fibers, or hydrophobic polymer pellets.    -   The aforementioned hydrophilic porous media may facilitate water        diffusion or flow within the plane.    -   The hydrophilic porous media may act as a buffer between the        catalyst layer and the membrane. Its thickness may range from        about 30 micrometers to about 1 millimeter, depending on the        porosity and hydrophilicity.    -   Any hydrophilic polymer material may be utilized to create the        hydrophilic porous media.    -   The ion exchange membrane may be a cation exchange membrane, an        anion exchange membrane, or a bipolar membrane.    -   In operation of the aforementioned membrane electrode assembly,        electrons may be conducted through the electron-conductive PRM        GDL, while gas reactants and products may be transported via the        intrinsic PRM GDL hydrophobic channels. Water may originate from        the anode and may cross the ion exchange membrane to the cathode        side. The hydrophilic porous media may enable water transport        within the plane. The water barrier layer may restrict water        diffusion, allowing only a portion of the water to enter the        catalyst layer. A significant portion of the water may escape        through the water pressure relief channel in the PRM GDL.    -   An alternative MEA design may be based on a non-conductive PRM        GDL that comprises four layers and one optional layer (not        including the anode). These five layers may be as follows: 1)        non-conductive PRM GDL, 2) cathode catalyst layer, 3) carbon        conductive layer, 4) optional hydrophilic porous media, and 5)        ion exchange membrane.    -   The non-conductive PRM GDL and the cathode catalyst layer        together may form the PRME.    -   The aforementioned carbon conductive layer may facilitate        electron conduction.    -   The carbon conductive layer may be made of materials, such as        carbon paper, carbon disk, or carbon foam.    -   The thickness of the carbon conductive layer may range from        about 30 micrometers to about 2 millimeters.    -   Conductive carbon particles or carbon blacks may be added to the        carbon conductive layer to reduce its porosity and enhance        in-plane electron conductivity.    -   The conductive carbon particles may be treated with hydrophilic        polymers to make them hydrophilic, allowing water to flow or        diffuse within the plane.    -   The carbon conductive layer may serve the dual functions of        electron conduction and water buffering, making the hydrophilic        porous media optional in this design.    -   The ion exchange membrane may be a cation exchange membrane, an        anion exchange membrane, or a bipolar membrane.    -   The working mechanism may involve the gas diffusion layer (GDL)        primarily serving as the pathway for gas diffusion while        electrons conduct from the electrode's edges, passing through        the carbon conductive layer to reach the catalyst layer. Water        transport within the carbon conductive layer may follow a        similar process as described above in the hydrophilic porous        media and may eventually escape through the water pressure        relief channels in the PRME.

The following examples are given for illustrative purposes only and arenot meant to be a limitation on the invention described herein or on theclaims appended hereto.

Example 1

A stability test was performed on a membrane electrode assembly, likethat shown in FIG. 13 , that included a 5 cm² cathode including anon-conductive gas diffusion layer having water drainage or pressurerelief channels in accordance with the present invention. The testinvolved feeding a dry CO₂ stream at a flow rate of 100 standard cubiccentimeters per minute (sccm) to the cathode side, while supplying 1 MKOH at a flow rate of 0.3 mL/min to the anode. A current density of 300milliamperes per square centimeter (mA cm⁻²) was applied, and the cellvoltage was maintained at approximately 3.25 volts for a duration of 75hours. Throughout the test, the C₂H₄ selectivity remained stable ataround 35%. The results of such testing are found in FIG. 15 .

Example 2

A stability test was performed on a membrane electrode assembly, likethat shown in FIG. 2 , that included a 5 cm 2 cathode comprising aconductive gas diffusion layer having water drainage or pressure reliefchannels in accordance with the present invention. The C₂H₄ Faradaicefficiency improved to around 40% and remained stable over 100 hours.The test was under a current density of 300 mA cm⁻². The results of suchtesting are found in FIG. 16 .

The embodiments of the present invention described above are intended tobe merely exemplary and those skilled in the art shall be able to makenumerous variations and modifications to it without departing from thespirit of the present invention. All such variations and modificationsare intended to be within the scope of the present invention.

What is claimed is:
 1. A gas diffusion electrode suitable for use in acarbon dioxide electrolyzer, the gas diffusion electrode comprising: (a)a gas diffusion layer, the gas diffusion layer comprising a poroushydrophobic structure having a pair of opposing sides, the gas diffusionlayer comprising a set of one or more pores designed for gas transportthrough the gas diffusion layer and a set of one or more openingsdesigned for water drainage through the gas diffusion layer, wherein theone or more pores are comparatively smaller in size and the one or moreopenings are comparatively larger in size; and (b) a catalyst layer, thecatalyst layer being disposed on one of the pair of opposing sides. 2.The gas diffusion electrode as claimed in claim 1 wherein the catalystlayer does not cover the one or more openings of the gas diffusionlayer.
 3. The gas diffusion electrode as claimed in claim 1 wherein thegas diffusion layer is electron conductive.
 4. The gas diffusionelectrode as claimed in claim 3 wherein the gas diffusion layercomprises a material selected from the group consisting of a metal orcarbon mesh, woven or felt material, paper, or disc, a porous graphitefilm, a metal foam, a metal sinter, and combinations thereof.
 5. The gasdiffusion electrode as claimed in claim 4 wherein the gas diffusionlayer further comprises a hydrophobic coating.
 6. The gas diffusionelectrode as claimed in claim 1 wherein the gas diffusion layer iselectron non-conductive, the gas diffusion electrode further comprisingan electron conductive layer.
 7. The gas diffusion electrode as claimedin claim 6 wherein the gas diffusion layer comprises one of ahydrophobic porous polymer membrane, a hydrophobic polymer-wovenmaterial or paper, and combinations thereof.
 8. The gas diffusionelectrode as claimed in claim 7 wherein the gas diffusion layercomprises one of a porous ePTFE (expanded polytetrafluoroethylene)membrane and a porous PVDF (polyvinylidene difluoride) membrane.
 9. Thegas diffusion electrode as claimed in claim 6 wherein the electronconductive layer is a porous structure made of at least one of carbonand a metal.
 10. The gas diffusion electrode as claimed in claim 1wherein each opening of the set of one or more openings extends entirelythrough the gas diffusion layer in a direct fashion from one of theopposing sides to the other of the opposing sides.
 11. The gas diffusionelectrode as claimed in claim 10 wherein the set of one or more openingscomprises a plurality of openings that are uniformly arranged.
 12. Thegas diffusion electrode as claimed in claim 10 wherein the set of one ormore openings comprises a plurality of openings that are randomlyarranged.
 13. The gas diffusion electrode as claimed in claim 10 whereinthe set of one or more openings comprises a plurality ofregularly-shaped openings.
 14. The gas diffusion electrode as claimed inclaim 13 wherein the regularly-shaped openings are circular openings.15. The gas diffusion electrode as claimed in claim 14 wherein thecircular openings have a diameter that is larger than about 30 μm. 16.The gas diffusion electrode as claimed in claim 13 wherein theregularly-shaped openings are square openings.
 17. The gas diffusionelectrode as claimed in claim 13 wherein the regularly-shaped openingsare rectangular slits.
 18. The gas diffusion electrode as claimed inclaim 17 wherein the rectangular slits are intersecting lines arrangedin a rectangular grid pattern.
 19. The gas diffusion electrode asclaimed in claim 17 wherein the rectangular slits are non-intersectinglines arranged in a rectangular grid pattern.
 20. The gas diffusionelectrode as claimed in claim 10 wherein the set of one or more openingscomprises a plurality of irregularly-shaped openings.
 21. The gasdiffusion electrode as claimed in claim 20 wherein theirregularly-shaped openings are slits.
 22. The gas diffusion electrodeas claimed in claim 10 wherein the set of one or more openings comprisesa plurality of slits and wherein the slits have a length that exceedsabout 1 millimeter.
 23. The gas diffusion electrode as claimed in claim1 wherein the gas diffusion layer consists of a single piece ofmaterial.
 24. The gas diffusion electrode as claimed in claim 1 whereinthe gas diffusion layer comprises a plurality of distinct piecesseparated from one another by spaces and wherein the spaces form the oneor more openings of the gas diffusion layer.
 25. The gas diffusionelectrode as claimed in claim 24 wherein the plurality of distinctpieces are arranged uniformly in a single plane in a rectangular gridpattern.
 26. The gas diffusion electrode as claimed in claim 24 whereinthe plurality of distinct pieces are arranged randomly in a singleplane.
 27. A membrane electrode assembly suitable for use in a carbondioxide electrolyzer, the membrane electrode assembly comprising: (a) anion exchange membrane, the ion exchange membrane having an anode sideand a cathode side; (b) an anode coupled to the anode side of the ionexchange membrane; and (c) the gas diffusion electrode of claim 1coupled to the cathode side of the ion exchange membrane.
 28. Themembrane electrode assembly as claimed in claim 27 wherein the gasdiffusion layer is electron conductive, the membrane electrode assemblyfurther comprising a water barrier layer positioned between the catalystlayer and the ion exchange membrane.
 29. The membrane electrode assemblyas claimed in claim 28 wherein the water barrier layer is a porousstructure comprising one or more materials selected from the group ofhydrophobic polymer particles, porous hydrophobic polymer particles,hydrophobic polymer fibers, and hydrophobic polymer pellets.
 30. Themembrane electrode assembly as claimed in claim 28 wherein the waterbarrier layer has a thickness in a range of about 200 nm to about 1 mm.31. The membrane electrode assembly as claimed in claim 28 wherein thegas diffusion layer is electron conductive, the membrane electrodeassembly further comprising a water capillary membrane positionedbetween the catalyst layer and the ion exchange membrane.
 32. Themembrane electrode assembly as claimed in claim 31 wherein the watercapillary membrane is a porous and hydrophilic structure having athickness in a range of about micrometers to about 1 millimeter.
 33. Themembrane electrode assembly as claimed in claim 31 further comprising awater barrier layer positioned between the water capillary membrane andthe catalyst layer.
 34. The membrane electrode assembly as claimed inclaim 27 wherein the gas diffusion layer is electron non-conductive, themembrane electrode assembly further comprising an electron conductivelayer positioned between the catalyst layer and the ion exchangemembrane.
 35. The membrane electrode assembly as claimed in claim 34wherein the electron conductive layer is a porous structure comprisingone or more materials selected from the group of carbon, copper, iron,stainless steel, silver, gold, nickel, aluminum, molybdenum, zinc,titanium, brass, and a metal alloy.
 36. The membrane electrode assemblyas claimed in claim 35 wherein the electron conductive layer has athickness in a range from about 30 micrometers to about 2 millimeters.37. The membrane electrode assembly as claimed in claim 34 furthercomprising a water capillary membrane positioned between the ionexchange membrane and the electron conductive layer.
 38. A carbondioxide electrolyzer, the carbon dioxide electrolyzer comprising: (a)the membrane electrode assembly as claimed in claim 27; and (b) avoltage source, the voltage source operatively coupled to the membraneelectrode assembly.